The present invention relates to a method for cutting or shaping sheets of brittle materials into desired configurations or geometries utilizing a laser-scribing technique. The present invention has particular applicability in laser-scribing brittle non-magnetic sheets along curvilinear paths to produce substrates for use in the manufacture of magnetic recording media.
Two techniques are conventionally employed for cutting or shaping a sheet of brittle material, such as a glass, amorphous glass, glass-ceramic or ceramic material, to form a sheet or substrate with a desired configuration or geometry. A first such conventional method involves mechanical scribing of the sheet employing a hard device, such as a diamond tip, to score the surface of the brittle material, which is then broken along the score line or pattern. The second of such conventional techniques involves laser-scribing. Currently employed laser-scribing differs from traditional high power (i.e.,  greater than 1 KW) laser-drilling/cutting and utilizes a lower power (i.e.,  less than 500 W) for achieving scribing with less material removal and better edge quality subsequent to breaking/separation. Such laser-scribing typically utilizes a continuous wave (xe2x80x9cCWxe2x80x9d) laser, such as a CO2 laser, to heat a localized zone of a brittle material, such as an amorphous-glass sheet (similar to float glass), up to a temperature below the softening point of the material, and then immediately quenching the heated zone by applying a coolant, e.g., a gas such as air or a liquid such as water.
In a typical process for laser-scribing an amorphous glass sheet, the output beam of a CW CO2 laser, or a high frequency pulse repetition rate CO2 laser, is re-shaped into a beam with an elongated spot shape, which beam is utilized in an unfocussed manner for locally heating the glass. The locally heated zone is then chilled by spraying cool air or an air/liquid (e.g., air/water) mixture. When the localized heating/cooling process starts from a small surface defect or micro-crack made in the glass, e.g., by a means of a mechanical scriber or indenter, or by application of suitable laser pulses, the defect or micro-crack propagates to form a scribing line due to the combination of localized heating-quenching which initiates tiny surface cracks arising from compression-tension stress effects. The sheet of material is then separated, i.e., broken, along the scribing line by applying an external thermal or mechanical stress.
A conventional laser-scribing technique utilizing a low power CO2 laser is disclosed by Kondratenko in U.S. Pat. No. 5,609,284, wherein an elliptical target area is impinged with a beam of coherent radiation along the intended direction of the crack, while a stream of fluid coolant is directed at a point on the heated surface on the intended line of the crack. U.S. Pat. No. 6,259,058 B1 to Hoekstra discloses a modification of U. S. Pat. No. 5,609,284 wherein dual laser beams are utilized after cooling in order to assist separation along the laser-scribing line. Allaire et al. in U.S. Pat. No. 5,776,220 disclose a laser-scribing technique for brittle materials wherein the laser spot has an extremely elongated elliptical shape such that its major axis is greater than 20 mm to enable rapid scribing.
Conventional substrates for use in manufacturing magnetic recording media include various brittle materials, such as glasses, ceramics and glass-ceramics. In order to form annular disk-shaped substrates suitable for use in magnetic and/or magneto-optical (MO) recording media, two circular scribings must be performed with high precision, one defining the outer diameter (e.g., ranging from about 65 to about 95 mm, such as 84 mm) and one defining the inner diameter (e.g., ranging from about 20 to about 25 mm). However, applicability of current linear laser-scribing techniques, such as utilized with flat panels, to circular scribing for producing annularly-shaped substrates suitable for manufacture of disk-shaped magnetic and/or magneto-optical recording media, is limited, for at least the following reason: laser-scribing is very sensitive to variations of the glass material, including optical reflectivity of the surface, glass composition, surface and thickness uniformity, etc., resulting in that the CO2 laser-based scribing process requires very precise control of defect initialization, laser power distribution, and cooling stream. As a consequence, current laser-scribing technology of amorphous glass substrates is generally restricted to linear scribing.
Another drawback/disadvantage of conventional laser-scribing technology is associated with the methodology for separating/breaking the brittle substrate (e.g., of amorphous glass) subsequent to laser-scribing. Specifically, because of the nature of the localized heating/cooling of the laser-scribing process, and due to the formation of a compression layer on the surface of the amorphous glass sheet, the propagation of micro-cracks during the laser-scribing process occurs in the layer nearest the glass surface. As a consequence, the scribe line provided by a single laser beam at the surface of a glass surface is insufficiently deep, and application of additional mechanical force to the glass sheet is typically required during the laser-scribing process or subsequent thereto, disadvantageously resulting in edge defects, residual stresses, increased risk of cracking resulting in product loss (i.e., low yield), reduced product throughput, and poor cost-effectiveness arising from a requirement for complicated, thus expensive, processing.
In view of the above-described disadvantages, drawbacks, and difficulties associated with utilization of laser-scribing technology for forming curvilinear-shaped brittle sheets, conventional practices for cutting/shaping brittle glass substrates for use in magnetic recording media involve the use of diamond tips to induce mechanical stresses. However, the required high tolerances are not fully satisfied by mechanical scribing. Moreover, it is necessary to perform extensive polishing and lapping after the mechanical scribing, thereby significantly increasing manufacturing costs.
Thus, there exists a need for a method and apparatus for laser-scribing a brittle substrate along a curvilinear path, particularly a substantially circular path. There exists a particular need for an apparatus and methodology for laser-scribing brittle materials, such as glasses, ceramics and glass-ceramics, along substantially circular paths to form annular disk-shaped substrates for use in manufacturing magnetic and MO recording media.
The present invention, therefore, addresses and solves the above-described drawbacks, disadvantages, difficulties, and shortcomings of the conventional methodologies and instrumentalities for performing laser-scribing of brittle glass substrates, particularly when utilized for performing curvilinear laser-scribing of amorphous glass substrates suitable for use in the manufacture of disk-shaped magnetic and MO media. According to the invention, a UV-based laser-scribing process and apparatus is provided which is especially well-adapted for shaping glass sheets into annular-shaped disks with inner and outer diameters, which methodology and apparatus provide a simple, readily controllable manufacturing process with increased product throughput and cost-effectiveness.
An advantage of the present invention is an improved method of shaping a sheet of brittle material.
Another advantage of the present invention is an improved method of separating a sheet of a brittle material into portions by means of laser-scribing.
Yet another advantage of the present invention is an improved method of shaping a sheet of brittle material along concentric inner and outer circular paths to form an annular disk.
A further advantage of the present invention is an improved apparatus for shaping a sheet of brittle material.
A still further advantage of the present invention is an improved apparatus for separating a sheet of a brittle material into portions by means of laser-scribing.
A yet further advantage of the present invention is an improved apparatus for shaping a sheet of brittle material along concentric inner and outer circular paths to form an annular disk.
Additional advantages and other features of the present invention will be set forth in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from the practice of the present invention. The advantages of the present invention may be realized and obtained as particularly pointed out in the appended claims.
According to an aspect of the present invention, the foregoing and other advantages are obtained in pair by a method of separating a sheet of a brittle material into portions by means of laser-scribing, comprising steps of:
(a) providing a sheet of a brittle material having first and second opposing major surfaces separated by a thickness t;
(b) providing a source of laser energy adjacent one of the first and second surfaces, the laser energy source adapted for supplying a focussed beam of laser energy converging to a focal spot in a plane parallel to the one surface;
(c) irradiating the sheet of brittle material with the focussed beam of laser energy from the source, the plane of the focal point of the laser beam initially being positioned within the sheet just below the first surface thereof; and
(d) continuing said irradiating while simultaneously: (1) providing relative movement between the focal spot of the laser beam and the sheet to move the focal spot at a pre-selected rate along a pre-selected path extending over the first surface, and (2) moving the depth of the plane of the focal point at a pre-selected rate through a major portion of the thickness t of the sheet to form a micro-crack zone extending along the pre-selected path from just below the first surface to a shallow depth just below the second surface.
Embodiments of the present invention include the following further step of:
(e) separating the sheet along the pre-selected path.
According to embodiments of the present invention, step (a) comprises providing a sheet of a brittle material selected from the group consisting of glass, amorphous glass, ceramic, and glass-ceramic materials; step (b) comprises providing a pulsed UV laser source, e.g., a pulsed UV laser source having a pulse repetition rate from about 10 to about 100 KHz, a wavelength from about 196 to about 355 nm, and TEM 00 mode quality M2 less than 1.5; step (c) comprises positioning the plane of the focal spot of the laser beam at a minimum depth below the first surface which does not result in micro-crack zone formation at the first surface; step (d) comprises moving the focal spot of the laser beam along the pre-selected path while the sheet is maintained stationary, or step (d) comprises moving the sheet along the pre-elected path while the focal spot of the laser beam is maintained stationary; wherein: step (d) comprises providing relative movement between the laser beam and the sheet to move the focal spot of the laser beam along a pre-selected linear, curvilinear, or linear-curvilinear path, e.g., a circular path.
In accordance with embodiments of the present invention, step (d) comprises moving the depth of the plane of the focal spot of the laser beam by optical means, or by varying a distance between the source of laser energy and the first surface of the sheet.
According to preferred embodiments of the present invention, step (a) comprises providing a sheet of amorphous glass for use as a substrate for a hard disk magnetic or magneto-optical recording medium; step (b) comprises providing a pulsed UV laser source having a pulse repetition rate of about 20 KHz, a wavelength of about 355 nm, average power of about 3.5 W, and a focal spot diameter  less than 5 xcexcm; and step (d) comprises providing relative movement between the focal spot of the laser beam and the sheet along at least one circular path at a rate from about 40 to about 100 mm/sec. to form an about 20 xcexcm wide micro-crack zone extending for a depth within the sheet of from about 100 to about 100 xcexcm; wherein: step (d) comprises providing relative movement between the focal spot of the laser beam and the sheet along concentric inner and outer circular paths; and step (e) comprises separating the sheet along the concentric inner and outer paths to form an annularly-shaped disk.
Another aspect of the present invention is an apparatus for performing laser-scribing of a sheet of material, comprising:
(a) a mounting means for mounting a sheet of a material having first and second opposing major surfaces having a thickness t;
(b) a source of laser energy adapted for supplying a focussed beam of laser energy converging to a focal spot in a plane parallel to the first surface of the sheet of material;
(c) means for providing relative movement between the focal spot of the laser beam and the sheet for moving the beam at a pre-selected rate along a pre-selected path extending over the first surface of the sheet of material; and
(d) means for moving the depth of the plane of the focal spot of the laser beam at a pre-selected rate through a major portion of the thickness of the sheet of material from a depth near the first surface to a depth nearer the second surface.
According to embodiments of the present invention, the source (b) of laser energy is pulsed UV laser source having a pulse repetition rate from about 10 to about 100 KHz, a wavelength from about 196 to about 355 nm, and TEM 00 mode quality M2 less than 1.5, and according to certain embodiments of the present invention, source (b) of laser energy is a pulsed UV laser source having a pulse repetition rate of about 20 KHz, a wavelength of about 355 nm, average power of about 3.5 W, and a focal spot diameter  less than 5 xcexcm; and means (c) for providing relative movement between the focal spot of the laser beam and the sheet is adapted to provide a moving speed from about 40 to about 100 mm/sec.
In accordance with alternative embodiments of the present invention, means (c) for providing relative movement between the focal spot of the laser beam and the sheet comprises means for moving the focal spot along a pre-selected linear or curvilinear path while maintaining the sheet stationary; or means (c) for providing relative movement between the focal spot of the laser beam and the sheet comprises means for moving the sheet along a pre-selected linear or curvilinear path while maintaining the focal spot of the laser beam stationary; and means (d) for moving the depth of the plane of the focal spot of the laser beam at the pre-selected rate comprises optical means or means for varying a distance between the source of laser energy (b) and the first surface of said sheet.
A further aspect of the present invention is an apparatus for performing laser-scribing of a sheet of material having first and second opposing major surfaces, comprising:
(a) a source of laser energy adapted for supplying a focussed beam of laser energy converging to a focal spot in a plane parallel to the first surface of the sheet of material; and
(b) means for effecting relative movement between the focal spot of the beam of laser energy and the surface of the sheet along a pre-selected path at a pre-selected rate while simultaneously effecting movement of the depth of the plane of the focal spot of the laser beam at a pre-selected rate through a major portion of the thickness of the sheet of material from a depth near the first surface to a depth nearer the second surface.
Additional advantages and aspects of the present invention will become readily apparent to those skilled in the art from the following detailed description, wherein embodiments of the present invention are shown and described, simply by way of illustration of the best mode contemplated for practicing the present invention. As will be described, the present invention is capable of other and different embodiments, and its several details are susceptible of modification in various obvious respects, all without departing from the spirit of the present invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as limitative.